Scientists are stumped by a jump in cases of Powassan virus, a life-threatening tick-borne neurological disease
On paper, Powassan virus sounds like your basic nightmare. A tick-borne infection with no vaccine and no cure, it kills 1 in 10 people who get it and causes long-term neurological problems in half of reported cases. Fortunately, Powassan virus disease for decades had affected only about one person a year in the U.S.—most likely because it was typically transmitted from woodchucks to humans by a tick that rarely bites people.
So back in 1997, when Sam Telford, a professor of infectious disease and global health at Cummings School, found a genetically distinct strain of Powassan virus in deer ticks—the bloodsuckers notorious for spreading Lyme disease—he initially worried about its implications. But despite the fact that he found the virus in 1 out of every 100 deer ticks he sampled, at the time there were no reports of swelling in the brain (called encephalitis) in sites where Lyme disease was common. He and his fellow researchers surmised that people just didn’t come down with Powassan virus disease from the strain carried by deer ticks.
“It turned out we were wrong,” he said recently.
In 2008, an elderly immunocompromised woman died from the deer-tick-carried subtype of Powassan virus. Since then, two additional human cases have been attributed to the same strain: a patient in Maine who died, and a Massachusetts patient who survived severe encephalitis caused by the infection.
Within the last five years, increasing numbers of severe encephalitis cases attributed to Powassan have been reported from states in the upper Midwest and the Northeast—places where Lyme disease and deer ticks are endemic. Massachusetts has been a particular hot spot over the last several years. The state Department of Public Health has reported 13 cases since 2012, with most of them from Cape Cod and northeastern Massachusetts. “I’ve heard of four cases in the last 10 months,” Telford said. “We need to figure out why we are starting to see that.”
Telford said he suspects that there has been some sort of natural selection for a variant of Powassan virus that’s more easily transmitted by deer ticks to people and just so happens to also cause severe disease. Such is likely the case regarding the recent emergence of babesiosis—a rare and sometimes fatal tick-borne disease that attacks human red blood cells—in the northeastern U.S. “Babesiosis used to be a coastal disease,” explained Telford, whose laboratory has been studying this phenomenon. “Now it’s all over the place, from New Jersey up to Maine.”
Telford, director of the Tufts New England Regional Biosafety Laboratory, plans to study whether a similar scenario is playing out with Powassan virus. He and his research partner Gregory Ebel, an associate professor in Colorado State University’s Arthropod-borne and Infectious Diseases Laboratory, want to catalogue the genetic signatures and locations of various strains of Powassan found in ticks across the U.S. and to test each variant in mice to see which cause more severe neurological symptoms. By answering these questions, Telford said, “we can better analyze the potential for Powassan virus to emerge as a significant and sustained public health threat in North America.”
Clinical symptoms of Powassan virus begin with a high fever and progress to neurologic signs such as headache, vomiting, weakness, confusion, seizures and memory loss. “One person I interviewed who survived Powassan virus in the 1990s still had pinpoint pupils and could barely walk two years after infection,” said Telford.
One of the big questions about Powassan is the range of its effects. With the closely related West Nile virus, 80 percent of infected people have no symptoms. The Eurasian tick-borne encephalitis virus also has been asymptomatic in 80 percent of patients, Telford added, “but of those who have gotten sick, 2 or 3 percent have died, and the survivors have severe complications, including paralysis.”
All that said, it is extraordinarily unlikely for a person to encounter a tick carrying Powassan virus. There are 3,000 to 5,000 cases of Lyme disease reported to the Massachusetts Department of Public Health each year, but the greatest number of Powassan cases ever reported to the agency was the five noted last year, said Telford.
“It’s not tick-ageddon,” he said. “People should continue to enjoy the outdoors. Just protect yourself against tick bites.”
Genevieve Rajewski can be reached at firstname.lastname@example.org.